Apparatus:
A spectrometer, spirit level, sodium lamp, diffraction grating with clamping arrangement and eye-piece.
Observations and Calculations:
Least count of the spectrometer = Value of the smallest scale division
No of divisions on the Vernier scale
No. of lines ruled on the grating = 15000/inch (say) = 15000/2.54/cm Grating element (a±b) == 2.54/15000 = cm
| Order of spectrum(n) | Vernier | Reading for | An le of diffraction | Wavelength | |||||
| Left | Direct | Right | Left | Right | Mean | ||||
| 12 | |||||||||
RESULT:
The wavelength of sodium light = Ao
Ao = 10-8
Procedure:
Focus the eye-piece on cross-wires and focus the telescope and collimator for parallel rays of light, level the prism table with the help of a spirit level. Make other necessary adjustments of the spectrometer as already explained. Make the slit of the collimator narrow and symmetrical with respect to the telescope cross-wires. Find the Vernier constant and note the readings of both Vernier’s against graduated circular scale.
Turn the telescope to the left through 90̊. Fix the telescope. Place the grating G on the prism table so that it is perpendicular to the line joining two of the leveling screws L and N of the prism table. Allow light from collimator to fall on the plane side of the grating (the side having no rulings). Rotate the table till the light reflected from the grating falls on the telescope cross-wires. It now makes an angle of 45̊ with the axis of telescope. Look for the image of the slit, if it is not symmetrical with respect to horizontal cross-wires of telescope, then it can be made symmetrical by turning screws N and L, this makes the plane of the grating vertical.
Read the Vernier and rotate the table through a further angle of 45̊ in the direction of the arrow head as in diagram. The plane of grating is now set perpendicular to the incident light falling on its back (un-ruled surface). Fix the table and release the telescope.
Turn the telescope slowly to the left till the image of the slit appears. This will be the first order spectrum. See if the slit is symmetrical with respect to cross-wire with the diffracted image, if not; turn the third screw M to make it symmetrical. Make the vertical cross-wire coincident with the left edge of yellow line and read the position OA1.
Adjust the telescope to see the clear image along OO1 as in fig, and note the Vernier position for direct image. Now turn the telescope to the right of the central position and when the spectrum is visible in the direction OB, make the adjustment of cross-wire as before and read the position OB. Difference is the angle
Make similar measurements on the second order spectrum and measure the angle
.Read the number of lines per inch ruled on the grating. Let number be 15000/inch then grating element a+b =2.54/15000 cm. In the formula,
the value of n will be 1 and 2 for first and second order spectrum respectively.
Precautions:
1. The grating should be placed with its plane vertical.
- The ruled surface of the grating should riot be touched.
- All adjustments of the spectrometer must be correctly made.
- The ruled surface should face to-wards the observer.
- The light should fall on the whole of the grating surface.
VIVA VOCE
Q.1. What type of spectrum do you obtain with a grating?
Ans. It is a normal spectrum.
Q.2. Define diffraction of light?
Ans. The bending of light around sharp corners and edges called diffraction.
Q.3. What is grating element?
Ans. On a grating the width of the spacing is denoted by a and that of rulings by b and (a+b) is called grating element.
Q.4. What is the difference between interference and diffraction fringes?
Ans. The interference fringes are equally spaced and diffraction fringes not equally spaced.